1. Field of the Invention
The present invention concerns load bearing structures having a honeycomb type core and more particularly honeycomb type cores in which corrugated core strips are individually attached to opposing face members before interconnection.
2. Description of the Prior Art
Load bearing structures having a honeycomb type core are employed in a diverse number of applications ranging from the space shuttle to battleships. Generally these structures are employed where substantial structural strength and minimum weight are desired. A typical honeycomb core load bearing structure includes a number of corrugated ribbons generally disposed in side by side relationship to form the honeycomb core with two planar face members or sheets attached to the lengthwise edges of the ribbons and two or more edge members attached to the edges of the face members. Often these elements are attached to one another by various welding and brazing techniques. In a number of applications the structure bears a load along one or more edge members. The core ribbons are then generally oriented either perpendicular to or parallel to these load bearing edge members.
Honeycomb core load bearing structures assembled by various welding and brazing techniques can generally be classified into one of two broad categories based on differing methods of fabrication. In one category the core is pre-assembled by first welding a series of alternatingly curved ribbons together to form the core, attaching edge members along edges of the core, and subsequently attaching face members to opposing sides of the core. Load bearing structures of this type enjoy a secure bond between the core and edge member which is important when the structure is supported at its edges. The attachment of the core to the face members, however, is very difficult since the core is not accessible for welding once the face members are positioned for attachment. Failures in this type of honeycomb core load bearing structure typically result due to weaknessess in the bond between the core and the face members.
In the other category, the honeycomb core load bearing structure is assembled by first positioning the face members adjacent one another and then individually inserting and attaching core ribbon elements to the face members, typically by welding. Thus, the honeycomb core is assembled during attachment of the core to the face members. Edge members are subsequently attached to the edges of the face members. With this type of assembly method the core ribbons may or may not be welded together depending on the degree of structural strength desired for the manufacturing cost involved.
While individual welding of core ribbons to the face members provides a superior bond between the core and the face members, the subsequent inaccessibility of the core after positioning the edge members for attachment precludes welding the edge members to the core. When the edge members are used to support this type of structure, a substantial flatwise tension is experienced between the core and one of the face members. This tension acts to pry the face member away from the core. It has been determined that the greatest magnitude of flatwise tension is located immediately adjacent the load bearing edge member. Since the core is not directly attached to the load bearing edge member, the sole load path for this maximum tension is through the end portions of a core ribbon disposed immediately adjacent the load bearing member. This characteristic can cause either dynamic or fatigue failure of the structure by separation of the core from the face members in the immediate proximity to the load bearing edge member. This separation will subsequently propagate towards the center of the core. Typically the structure will fail through separation of a face member from core ribbon ends adjacent a load bearing edge member at loads approximately 65% of those which can be supported by the central portion of the structure.
Previous efforts to avoid failure in this type of honeycomb core structure due to the maximized flatwise tension at the ends of core ribbons disposed adjacent a load bearing edge member have focused primarily on providing thicker load bearing edge members and face members so as to dissipate a greater amount of the flatwise tension into inner portions of the core. This approach, however, provides only a limited increase in the strength of the structure at the cost of an undesirable increase in overall weight. Further, the maximum flatwise tension is still located at the core ribbon ends adjacent the load bearing edge member, resulting in a fatigue failure for a considerably smaller load than that which can be supported by the central portion of the structure.
Thus, there still exists a need for some manner of minimizing the flatwise tension applied to core ribbon ends in immediate proximity to a load bearing edge member.